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Gut Health x Behaviour x Performance

Poor performance? Erratic behaviour? It may be stemming from your horse's gut and diet! Diet can influence gut health, which can influence behaviour, which can influence performance, which can also influence gut health, and understanding these interactions is essential in achieving both optimal health and performance.

Interrelation is a common theme when it comes to equine health. For example, when thinking about hoof health, we must consider trimming practices, and the influence of diet, as well as management, environmental and postural factors. Additionally, when aiming to put weight on a horse, we cannot just consider calories in the diet - we have to consider dentition, whole body comfort, and factors impacting intake and expenditure, such as being low in the pecking order outside, mental wellness, overall health and gut health status. 

Equine science, which considers equine physiology, anatomy and behaviour, allows us to better understand these interactions so that we can strategically align our feeding and care practices to optimize both health and performance.

Connecting Normal Feeding Behaviour with Gut Physiology

Feeding behaviour has been reported to occupy 50-77% of the free-roaming horse’s time budget. Note that feeding behaviour includes foraging - watching your horse graze, you know grazing is tied to movement as they actively seek out the best piece of grass. As such, foraging behaviour constitutes a behavioural necessity in our horses. When this normal behaviour is not permitted, the horse will find an alternative behaviour as an attempted solution, which manifests as stereotypical behaviours. A study comparing the effect of free-choice and restricted hay in cribbing horses found that cribbers will increase their cribbing frequency by 200% when hay is restricted, compared to when they have free-choice hay available.

Behaviour also mirrors a physiological need. As the horse is adapted to eat little and often, the equine stomach secretes acid continuously. However, this constant acid secretion puts the horse at risk for gastric ulcers, especially when combined with factors that increase acid exposure to the squamous area of the stomach, such as exercise and lack of forage availability. 

CONCERN? Horses are adapted to spend most of their time grazing, which is reflected in both their mental and physical functions. 

Effect of Diet on Gut Health 

A horse’s diet can have incredibly significant impacts on their gut health, especially if their diet does not match their functional anatomy. As grazers, horses are adapted to eat a fiber-based diet, and are very efficient at doing so! This is due to a symbiotic relationship with microbes in their hindgut. While mammals lack the ability to break down the structural cell wall components of plants, microbes in the horse’s hindgut can! Horses have a vast microbe population in their hindgut, consisting of bacteria, protozoa and fungi. With regards to bacteria, the majority are meant to break down fiber, which convert fibrous carbohydrates (structural carbs) to volatile fatty acids for energy..There is also smaller population of bacteria which can hydrolyze rapidly fermentable carbs, such as starches, to lactate and propionate. 

These microbes break down these fibers from forage into short-chain fatty acids, which are absorbed in the hindgut and used for energy! 

Meanwhile, grains provide more starch, a hydrolyzable carbohydrate that is digestible in the small intestine. Inclusion of grains in the diet increases propionate and lactate production at the expense of acetate. This is transformed to glucose and transported into the blood, where it is transported and stored or used as fuel. Increasing the proportion of grain in the diet increases risk of hindgut acidosis, where grain escapes small intestine digestion and reaches the hindgut, where it is fermented to produce lactic acid. This change in pH kills off the good microbes and can cause GI upset (colic) and in some cases, laminitis.

Some performance horses are fed high-starch diets in effort to meet energy requirements. Horses have a limited capacity for digesting starch, and high quantities can cause hindgut disturbances, cecal epithelial damage, increased endotoxins in plasma, and dehydration in the colon. These effects can result in other health issues such as colic, hindgut ulcers, laminitis, and immune suppression.

The most effective way to maintain proper GI health and microbiome balance is to avoid hindgut acidosis by feeding high fiber, low/controlled starch diets, while allowing normal feeding behaviour such as slow and frequent consumption of forage. A horse can consume 1.5-3.0% of their BW on a DM basis, with the average voluntary hay intake around 2% BW. It’s recommended that horses consume a minimum of 1.5-2.0% of their body weight in forage on a dry matter basis (without the weight of feed moisture included). However, as many common practices and activities such as competing can increase risk of GI upset, nutritional strategies which support digestive health may help protect the horse from GI upset.

How Does Exercise Impact Gut Health?

There are many factors which can impact the gut microbiome, such as diet, physical condition, use of medications and feed ingredients, as well as exercise!

Exercise is a physiological event: the body undergoes physiological processes influenced by physical effort in order to support the required physical output (ie, increased heart rate). While these responses are normal and necessary to support exercise, they can impact your horse’s gut health.

To support exercise, circulation is increased to muscles and other vital organs (heart, lungs, etc). Circulation is reduced in the abdominal cavity, slowing down processes such as digestion.

CONCERN? Reduced digestibility. Research has yet to determine if this has practical significance.

Due to the significant impact that exercise has on other physiological processes, there has been interest in whether exercise can impact the gut microbiome as well. There is currently a limited amount of quality research available in this area, but preliminary data suggests that exercise seems to alter gut microbial composition.

  • A 2021 study (Gorniak et al, 2021) reported increased levels of Bacteroidetes and Firmicutes, the two most abundant phyla in the equine hindgut, in feces post-exercise, compared to pre-exercise.

  • An earlier study (Janabi et al, 2016) also reported bacteria changes associated with exercise training, compared to a control group, however these changes were transient, and reversed throughout the 12 week study. 

Levels and ratios of Bacteroidetes and Firmicutes have been hinted at having relevance in intestinal condition and health, however existing research suggests that these bacteria phyla alone are not sufficient to evaluate intestinal health in horses (Park et al, 2021). 

CONCERN? Altered microbe ratios and dysbiosis are associated with health issues and colic. More research is required to determine the significance of these changes on equine gut health.

Exercise increases the risk of acid splash in the stomach. The abdominal cavity contracts during exercise, increasing gastric pressure. This pushes gastric contents up, towards the sensitive, non-glandular section of the stomach. As acid is produced from the glandular section, it has natural protection against ulceration. Meanwhile, the non-glandular section is sensitive to harm from acid exposure.

CONCERN? Acid splash can cause discomfort, pain, and can contribute to the development of gastric ulcers.

The influence of performance on gut health is not limited to the effects of exercise alone - it is important to remember that performance horses are often exposed to many stressful situations or challenging management practices, which add further risk for gut health. 

  • For instance, transportation is associated with the occurrence of GI issues. Ulceration can occur with just 1 transport trip, even in horses who had no signs of ulceration prior to transport!

  • Environmental stress also increases risk of ulcer development, which is relevant for show horses who are exposed to new environments frequently.

  • Stall confinement is a factor in gastric ulcer development. Show horses may be stabled more both at shows and at home. Lack of social exposure to other horses can also increase stress, and thus, risk of GI upset.

  • Horses are not designed to be without feed for long periods of time as their stomach produces acid continuously. Feeding frequency & time without feed increases risk of GI issues. Sport horses may spend time without feed during long show days, or if they are stabled without free-choice hay. Fasting before transport also increases risk of ulcer development.

Performance horses who experience gut discomfort may have more undesirable behaviours, contributing to reduced performance. While exercise itself can impact gut health, performance horses have multiple potential risk areas to their gut health, so it is imperative to keep in mind that undesirable behaviour in performance horses may be a reflection of gut discomfort.

Gut Health & Behaviour

“Many behavioural problems are organic in origin.” - Miller (2001)

Behaviour is information, and can give us valuable insight on our horse’s health. Increased anxiety and aggression are associated with digestive discomfort in many species. However, behavioural signs of pain in horses often go unnoticed or are discounted with anthropomorphic blanket statements of the horse’s temperament.

A horse’s gut health status can impact normal behaviour in numerous ways.

  • Hindgut acidosis can cause the horse to stop eating, which we know, is significant for an animal who is designed to eat little and often.

  • Hyperactivity can arise from gut discomfort, as the horse attempts to ‘escape’ the pain. High starch diets result in higher reactivity than high fiber diets.

  • Destrex et al (2015) observed that the time horses spent being wary of their environment was positively correlated (as one increases, so does the other) with altered microbiome concentrations during novel object response tests when horses were on a high-starch diet. Increased vigilance behaviours (ie, being ‘spooky’) might be an indicator of intestinal discomfort. 

  • High grain diets are also associated with stereotypical behaviours (aka “stable vices). Cribbing has been found to intensify following grain feeding, while weaving peaks prior to grain feeding, especially when the horse has no other food available. 

  • A retrospective study done by Millares-Ramirez and Le Jeune (2019) described common clinical diagnoses of horses who were described to be “girthy” at large animal clinics from 2004-2016. Ulcers, orthopedic pain and miscellaneous pain were the largest diagnoses of horses who had been described as girthy as part of their intake.

    • Due to the retrospective nature of the data, not all horses were gastroscoped. 92% of girthy horses who were gastroscoped had ulceration present. As such, ulcers may be an under-represented contributor to girthiness in this study.

  • Studies have reported that gastric ulcers negatively impact physiological measures of athletic performance in racehorses. Racehorses who are treated for ulcers had improved ratings & earnings compared to before treatment.

As evidenced in the Ridden Horse Pain Ethogram, many signs of poor performance under saddle are associated with pain. As such, gastric ulcers and other digestive discomfort can contribute to poor performance in sport horses, and should be considered when troubleshooting performance issues.


What does this teach us? Behaviour is information. Girthiness is associated with pain & discomfort. Increased ‘aggressive’ behaviour is a pain response. Poor performance in sport horses may speak to the many risks present for performance horse gut health.

But do not let the interaction between gut health, behaviour and performance scare you! Instead, being aware of these interactions provide us with a great opportunity to identify issues and remedy them - noticing these signs of discomfort helps give us a starting point towards helping the horse out of the discomfort. Information is something that we can do something with, as long as we remember there is often a physiological reason behind undesirable behaviour. Animal science & health is incredibly complex, so being aware of these interplays helps us strategically optimize these interactions in our care & management. 


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